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"The next election campaign will be tough for Labor and it needs the next generation coming through," he told the ABC on Wednesday.

"It doesn't need me staying there because I can stay there. It needs the next generation, it needs people with a 30 or a 40 in front of their age."

The former Woodside executive's decision came as a shock because he fought so hard against left-wing unions, led by the Maritime Union of Australia, that sought to unseat him last year.

First elected in 2007, he believes Labor will keep its WA seats - and possibly even pick up some more - but admits Mr Shorten is unlikely to prevail over Prime Minister Malcolm Turnbull.

"I think it's highly unlikely that an opposition, at any time, wins a first-term election," he said.

I think it's highly unlikely that an opposition, at any time, wins a first-term election

Mr Shorten said Mr Gray was only pointing out the obvious.

"Gary is correct in saying that Labor is the underdog," he told reporters in Perth. "I don't think Gary is telling you something that you couldn't have found by Googling the history."

But he said he was not in politics to be an honourable loser: "We are in it to win it. I acknowledge this is a hard election, but millions of Australians need a strong Labor Party offering policies which help the working class and the middle class."

Mr Gray is respected across the aisle, with Environment Minister Greg Hunt saying he was "genuinely sorry" to see him go and Resources Minister Josh Frydenberg calling him a successful minister and strong advocate.

The decisions by Ms Parke and Ms MacTiernan also came as a surprise. Ms Parke said she wanted to spend more time with her family while Ms MacTiernan - who was only elected in 2013 - said she was not making enough of a difference to warrant the sacrifices of the job.

Mr Gray - who took over Brand from former opposition leader Kim Beazley - said Ms MacTiernan's decision helped him make up his own mind to leave. He has not said what he intends to do now.

The pre-selection dramas will be on Mr Shorten's agenda during his talks in Perth this week.